One of the major issues related to current surveying methods using these electromagnetic imaging techniques is the effect of steel casing on the fields measured by borehole sensors. This has been detailed in the literature review contained in U.S. Pat. No. 6,294,917, which is hereby incorporated by reference herein, which also provides a method for calculating ratios of data to remove the effects of steel casing. Problems are also associated with near-surface heterogeneity when any sources or sensors are located on the earth-surface. Examples of these effects in surface EM measurements can be found for magnetotelluric measurements in Torres-Verdin, Carlos, and Bostick, Francis. X., 1992, Principles of spatial surface electric field filtering in magnetotellurics: Electromagnetic array profiling (EMAP), Geophysics, 57, 603-622, and for transient EM measurements employing a grounded source in Strack, K. M., 1992, Exploration with Deep Transient Electromagnetics, Elsevier, Amsterdam, and Newman, G. A., 1989, Deep transient electromagnetic soundings with a grounded source over near surface conductors: Geophys. J. Int., 98, 587-601. Because near surface heterogeneity, as well as cultural structures such as pipelines, metal fences, and buildings, can affect STB or BTS measurements, methods to remove these effects will help to improve image accuracy.
Another method is described in U.S. Pat. No. 6,393,363, which details a numerical modeling based method for correcting data collected in steel cased wells to produce a ‘calibrated’ data set free of casing effects. However, this method relies on prior knowledge of the geoelectric structure surrounding the wells, and the degree of error in the calibrated data that is introduced by incorrect model assumptions is unknown. Although the ratio method outlined in U.S. Pat. No. 6,294,917 has the draw back of decreasing the signal-to-noise ratio of the data that are used for imaging, it makes no assumptions about the structure that the user is trying to image.
Therefore, it is a desire to provide an improved system and method for reducing distortion caused by near-surface heterogeneity and removing the effects of steel casing in wellbores.